1. Field of Invention
The invention relates to an infinitely variable transmission that generates torque circulation by combining a continuously variable transmission apparatus with planetary gears. In particular, the invention relates to an infinitely variable transmission that uses a belt type continuously variable transmission apparatus well suited for being mounting on a vehicle and that creates a neutral state with an output member having no rotation by means of the torque circulation.
2. Description of Related Art
Automatic transmissions integrating a belt type continuously variable transmission apparatus (CVT) have been receiving attention as vehicle transmissions due to demand for improved fuel consumption rates and improved driving performance
As disclosed in Japanese Laid-Open Patent Publication 6-331000, an infinitely variable transmission comprises a continuously variable transmission mechanism, a constant speed mechanism and a planetary gear system wherein torques from said continuously variable transmission mechanism and from said constant speed mechanism are combined in said planetary gear system and a torque circulation is generated in the continuously variable transmission mechanism in order to increase the range of transmission ratios. The infinitely variable transmission transmits engine output to a carrier through the constant speed mechanism as well as transmitting to a sun gear through the continuously variable transmission mechanism and a first (low) clutch. In this state, a torque circulation is generated in the continuously variable transmission mechanism and the transmission ratio of the infinitely variable transmission as measured by the ratio of the rotation of the output shaft over input shaft rotation changes from reverse to a large forward ratio (U/D) to a small forward ratio (O/D) with the transmission ratio of the continuously variable transmission mechanism changing from small (O/D) to large (U/D). By releasing said first clutch or a one-way clutch and by engaging a second (high) clutch, the rotations of the continuously variable transmission mechanism are directly transmitted to the output shaft, and the transmission ratio of the infinitely variable transmission changes from large (U/D) to small (O/D) with the transmission ratio of said continuously variable transmission mechanism changing from large (U/D) to small (O/D).
With an infinitely variable transmission that executes the above mentioned torque circulation, a gear neutral position where the number of rotations of the output shaft becomes 0, can be calculated in theory by assigning a predetermined value, which is determined from the gear ratio of the planetary gear mechanism, to the transmission ratio of the continuously variable transmission mechanism. Hence, in theory, the mechanism works without a starting apparatus such as a starter.
However, with the infinitely variable transmission discussed above, although in theory a gear neutral point may exist, the output torque diverges rapidly towards infinity in the vicinity of the neutral point, causing the output torque to fluctuate greatly with a slight deviation from the predetermined transmission ratio which makes the continuously variable transmission mechanism approach the above-mentioned neutral point. Hence, in reality, it is very difficult to establish and maintain the pulley ratio of the continuously variable transmission at a target transmission ratio to achieve the above-mentioned neutral point by controlling oil pressure and the like, to regulate the shaft force being applied to both pulleys of the belt type continuously variable transmission apparatus.